Quirks of Mikuni/Solex's and Webers

[Billzilla]

There's been some debate of how good the Mukuni/Solex's that came with 2TG's are compared to the typical DCOE Weber, and so which one is a better thing.
I'd like to add my experiences with both of them, and they go back nearly twenty years to when I got my 2TG in 1984.
I was one of the early modifyers in Australia to fit a 2TG to a Corolla back then as they were pretty thin on the ground and for the first year or two I can only remember seeing maybe a handful. The better ones were build by Jimmy Bertram of GP Cars and were fitted to Group G rally TA-22's, and they make in excess of 170hp at the wheels. However, they were bored out to over 1700cc to do that. They were the most powerful 2TG's around and please note that for later.
Anyway, I left mine stock for a couple of years, (albeit cleaning up the poor inlet manifold) then added some bigger cams in about 1986.
Around that point, the engine started to run badly. It's a long story but I ended up finding that three of the auxiliary venturi in the Mukuni's had fallen out and were rattling around inside the throats of the carbies, and the fourth was loose & about to fall out. Two were damaged beyond repair, and one was kinda workable.
In all of Australia, there was one in stock and they emergency back-ordered two for me from Japan. They took a week or two arrive.
Anyway, I did my best to try to keep the damn things locked down and not fall out, but not matter how tightly I did up the locating screw and locked it in place they still kept coming loose for years to come. I had to keep checking them every couple of months to be sure.
Since they did this as they came from the factory (only a few years old, they were still relatively new back in the mid-80's) I assumed either I was unlucky or it was a bad design.
Wind the clock forwards another couple of years. A mate of mine bought a KE30 Corolla off me that had a good 4K I'd built-up in it, but he took that out and fitted the last of the carby-fed 2TG's into it. It had the later Mikuni's, the ones with the external adjusters for the float levels. He kept it absolutely stock, but again after about six months the auxiliary venturi's started to come loose. From memory he was able to tighten them with reasonable success, but he sold the car soon afterwards.
Wind the clock forwards to a few years ago. I'm in the Broadwater Carpark here on the Coast and there's a guy wtih an RA-40 and a 2TG in it. It wasn't running very well, and guess what? Yep, loose venturi's. I was able to tighten them with some tools he had, and it made the car much better.
On this very forum again there's been multiple cases of the same thing, with a memorable one being only a few months ago. A guy posted that his 2TG wasn't running very well and about half a dozen people had a guess as to what was wrong and although some were good guesses, none helped. And you guessed, I said "loose venturi's" and I was right.

So let's look back at this.
Pretty much every single Mikuni that I've come across from a 2TG has had that problem. It's not the odd one out, it's very common. That's not poor maintenance from the owner, it's just a poor design from Mr Mikuni. Hardly any other carbies fall apart like that, so it's not inaccurate to say that they aren't a good carby in that area. If you haven't had the venturi's come loose, then you're very lucky.

The other problem living with them is - although things may have changed so I'm wrong - is that back when I was fiddling with them it was pretty much impossible to get any bits for them. There was a very small number of places that had jets & emulsion tubes, but again the selection was limited. Things like 45mm versions or larger chokes were just not possible to get. Webers were, and still are, very easy to get parts for.


So what did the race & rally cars of the day do? They ditched the Mikuni's and fitted DCOE Webers. Fortunately, because Mr Mikuni copied the mounting flange of the DCOE Weber you could bolt Webers straight onto the 2TG manifold. With a little massaging a 45mm Weber could be used.
The rally boys (Stewie Reid, Jimmy Bertram, Greg UmSomeone, etc) all reckoned that simply bolting the Webers on and tuning them was worth about 10hp.
No doubt if more bits were available for the Mikuni's then better results could be had from them though. Certainly the (cold start) choke on them is a better thing than the Webers, for example. However, the Webers use a ring design for the auxiliary venturi and it is utterly relaible. Webers do not fall apart - They get out of tune and get very worn, but the internals do not rattle around inside them.
I'm thinking that perhaps part if this is due to the resin block that had the air balance path between the two throats on each Mikuni, as they had the carbies mounted solidly on the inlet manifold. Webers typically have the soft rubber mounts & spring washers that let them flx around a little ...

It's been mentioned that TRD recommended Mikuni's. I think this means little as it's likely just sponsor pressure to make them say that. In any case, the Mikuni's would have been modified (as are Webers) to work as required, and maintained far better than any street carby. I just can't see a major Japanese tuner recommending an Italian carburetor - That works the other way of course, has anyone seem any Italian teams using/recommending the Mikuni copy over the Webers? No, of course not.
And in any case, the proof for the Aussies was that the people that did all the winning ditched them for Webers.

So quirks of Webers.
They come with two types of floats, copper and plastic. In some cases the copper floats can develop a tiny little hole and so fill up with petrol and sink. So no matter how carefully you set the upper and lower float levels, the floats run low and so the engine runs rich. Much the same thing was done by many people who trailered their car to the track - Sometimes the flaot bowl would be empty of fuel and so the floats would be free to bounce up & down over the bumps. That would gradually make them alter the lower position for the float, so if you were unlucky enough again the engine would run very rich. The cure was to make sure the carbies were full of fuel before going anywhere.

Another good one was when fitting air filters over them. Sometimes they'd block off the float chamber breather hole at the front of the carby body, so the chamber wouldn't be able to breath properly and so let the fuel level settle proplerly. The other extreme was to leave the breather hole unfiltered, so dusty air could get in and fill the fuel bowl up with grit.
The DCOE's are good for lateral loads up to around 1G or so, but then they occasionally suffer from fuel splashing around the top of the emulsion tubes, which upsets the mixtures. The cure for cars that make more cornering G's than that was to fit towers to the cap that cover the air correction jets/emulsion tubes. The towers on the 45's on my racing car were a good 30mm tall and it'd do 2.2G's laterally no probs. It's easy to do with Webers as they have a round cap that's easy to make on a lathe, whereas the Mikuni's have a rectanugular one that'd be very difficult to fabricate.
The other nasty thing the Webers have that the Mikuni's don't is a very poor throttle return spring -In the Webers it's a short little spring tucked away behind a plate at the back of the carby. The first thing you do for just about any application is take that spring out and throw it away, as they have a very common tendency to break and sometimes jam the throttle open. External springs were the way to go for all Webers of that type. I can't remember how the Mikuni's worked, but I think it was a better arrangement.

So there ya go, some of my dealings with those pair of sidedraft carbies going back nearly 20 years.
Hope it helps.

(More if I think of it later)

[SRacin]

Thats a great read! Some very very handy information that may save alot of people some stress.

When I first starting playing with 2tg solex carbs on my GT TA22 nearly 10 years ago, guess what happened? Thats right the #3 venturi came loose I think the universal "soft mount" kits are an absolute must have, and your experience and knowledge confirms their need!

Cheers
Logan

[4agte]

Agree with the lateral loadings espc if you needed alot of throttle quickly

[river]

Hi,

Good post. It shows that both the Webers and the Solex have their nuances, strengths and weaknesses, and in reality, one doesn't seem to be better then the other when all these things are considered.

The Solex in the 18R-G also get loose venturis. I check them every few months and tighten them up if they're loose. It's not a hard job and as I enjoy tinkering with the car I find it a pleasurable task to do this sort of thing. It's one of the reasons why I like carbs - 'cos I like to tinker. If I didn't then I'd take the easy way out and put in a good ECU and fuel injection.

Although you mentioned that in Australia they used Webers, TRD won many races around the world using Solex. I don't know of any sponsorship that forced them to use these carbs.

Yes, parts are a bitch to get for the Solex, but they're a hardy carb that doesn't wear out many parts and a little bit of periodical preventative maintenance (ie checking/tightening the venturis) can see them last many years.

The Solex on my engine are now over 33 years old. They've been rebuilt once and all they required was new gaskets and diaphrams as the old ones were brittle from age. The car has always started and ran well in all conditions and I can't fault their operation or performance, as I've never been stranded or suffered major performance issues due to the carbs.

While I don't doubt that some people say that replacing their Solex with Webers gave them extra power, I'd like to see the graphs and other accurate details before accepting this as truth.

seeyuzz
river

[Billzilla]

While I don't doubt that some people say that replacing their Solex with Webers gave them extra power, I'd like to see the graphs and other accurate details before accepting this as truth.

As mentioned in the post, it was more a factor of being able to get the bits to tune them. The 2TG Mikuni's have very small chokes and you just couldn't get larger ones. It's possible to modify the existing ones, but ever so much easier to throw a set of Webers on and have any size choke you like.
From memory the Mikuni's have something like a 33mm or 34mm choke, and that's too small for good power from a good engine. Something more like 38mm or even bigger is needed.

[Hurricane]

yeah i would go the weber over the solex cos parts are easyer and cheaper to get and its easyer to tune and that, but in sayin that im inthe proses of puttin solexs on my 3T in my rolla, but only cos i have them hear and am goin to run injection in the near future so don't want to go and spend more money on carbys

[kingmick]

Top post Bill.
Bit of work in that one mate, id plus rep you but i have to spread the love first!

Yes, i like webber aswell but also holley

[nb86]

My experience has been the same, carbs are much the same, webercarbs and spares just much more readily available. One extra nuance of the webers is that after a few seasons of use with external return springs, the side bearings and seals will develop play and leak. They are fairly straight forward to replace though, So if you have idle and light load running problems, check the shaft for play.
They same is true of solex's, but the nylon bush they use is harder to replace, and harder to get hold of.

[Grega]

nice one bill. some good info there mate

one thing to mention - fuel frothing and fuel pressure supply. webers, mikuni and dellorto all dont like any sort of high(ish) fuel pressure - ~3.5psi tops. As has also been mentioned before softmount kits are absolutely mandatory to stop fuel frothing in the carbs (thru vibration) as are the lock springs and other paraphanalia to mount the carbs to the inlet manifold....

[Rodger]

Good post Bill.

Experience counts.

I have had nothing but good experiences with my Mikuni/Solexes. 44mm Mikunis are hard to get in Aus and I have two pairs and have seen/rebuilt another three pairs over the years so they are around.

I am happy just to support those with Solexes and run my 44s in competition but I like anything that runs sidedraught carbies.

Have to agree that the Weber can be tuned a bit better as there are more bits to change. For instance the Mikuni/Solex has a fixed size Pilot Air Jet, which can only be left as is or drilled bigger.

Regards

Rodger

[Billzilla]

Thanks guys.
(I still personally wouldn't use the Mikuni's though)

[Rodger]

BTW Bill

How many jet covers did you loose before wiring them up?

I suppose once you know the little secrets to keep any brand of carby at their best we then tend towards that brand.

Regrads

Rodger

[Billzilla]

BTW Bill

How many jet covers did you loose before wiring them up?

I suppose once you know the little secrets to keep any brand of carby at their best we then tend towards that brand.

Regrads

Rodger

None on either type of carby mate.
Though we always lock-wired the Webers on the racer anyway.

[Jonny Rochester]

I don't disagree, but a few other points:

I have not had any trouble with the venturis coming loose after using some thread locker or sealant on the screws. Fixed. But every year or so a main jet holder may come loose causing a misfire under load. Just do them up firmly with the correct large flat screwdriver.

I do not like Webbers with softmounts because, I have seen the O ring crack and cause a air leak. And I have seen people overtighten the mounting bolts causing a corner of the carby to snap off.

I HATE dodgy home made throttle cables and springs. They very rarely work well for any time. I have factory genuine toyota throttle linkages and springs on Mikunis (as much as posiable), and I don't have any problems. They DO NOT go out of tune. They stay put for years. Very reliable.

My setup:
3T-G with factory intake manifold, bolted to the head with homemade cardbourd gasket (wheetbix or similar), and either Permatex No.3 or motorbike crankcase sealant (fuel resistant). (Don't use silicon).
Factory heat insulator spacer and balance tube, with gaskets and Permatex No.3 or No.4, or that motorbike stuff.
I have early Solex Mikuni 40mm carbs (off TA22 or TE27 or early TE37 I guess), with 32mm big venturi. And I got factory throttle linkages for cable that are hard to find. Plus 4 redline horns mostly for looks. Drilled jets bigger to suit my big cams. Any unused holes are filled with Permatex No.4 (hardening).

I have noticed the standard mechanical fuel pump works fine, but I am using a electric one with no pressure regulator.

I use the cold start sometimes. In the morning, pull the cold start, 2 pumps of the throttle, turn the key and it starts every time.

I have the vaccum advance hooked up but it's not functional. Not enough vacuum to operate the diafram.

[Steve M]

Great thread Bill.

About the only issue we've found with the mikunis is that they flood when going over very corrigated roads. The fuel sloshes out of the float chamber breather and straight into the air correctors, like you mentioned Bill. It always pays to go over bumpy hills roads at full throttle just so it will keep going.

The really early 44/50mm mikunis from the 1960s (PHH-2 with two bolt jet caps) have a much better sealing arrangement that stops the fuel getting into the top of the air correctors. These carbies unfortunately are made from a very soft alloy and don't have mecu metal to bolt the venturis in through so the threads strip very easily.
The small/inner venturis also are thinner than other models and squish when they are done up. The gasket kits are really hard to find for the PHH-2. They are made by pirelli.

We've never had a venturi come loose on the 18RG.